Photoconductive potentiometer using variable transmittance control strips

ABSTRACT

A light control member in the nature of a photographic wedge having varying light transmittance characteristics along its length, is interposed between a light source and a photoconductive cell. The light transmittance characteristics are made to vary along the length of the control member in accordance with a predetermined function of resistance. Means are provided to position the control member so that various portions thereof are placed in the light path between the light source and the cell, thereby varying the light incident on the cell in accordance with the predetermined function. In this manner, the effective resistance of the cell can be controlled in accordance with the light transmittance variation along the length of the control member, this variable resistance being usable for a desired control function.

[ 5] Feb. 1,1972

United States Patent Clark 6 2 2 W a ye mm mm mm m m ME m 3 um [54] PHOTOCONDUCTIVE POTENTIOMETER USING VARIABLE TRANSMITTANCE CONTROL STRIPS Attorney-Sokolski & Wohlgemuth ABSTRACT A light control member in the nature of a photographic wedge having varying light transmittance characteristics along its length, is interposed between a light source and a photocon [22] Filed:

[2]] Appl. No.: 815,019

ductive cell. The light transmittance characteristics are made to vary along the length of the control member in accordance with a predetermined function of resistance. Means are pro- [52] US. [51] Int. Cl.

[58] Field vided to position the control member so that various portions thereof are placed in the light path between the light source 6M6 ZIM mm nu L 1 m ..I. m 1 l 2 I .0 5

References Cited UNITED STATES PATENTS 2,896,086 7/1959 Wunderman...................

and the cell, thereby varying the light incident on the cell in accordance with the predetermined function. In this manner, the effective resistance of the cell can be controlled in accordance with the light transmittance variation along the length of the control member. this variable resistance being usable for a desired control function.

3,072,795 l/l963 Badmaiefi............ 3,087,069 4/l963 Moncrieff-Yeates 3,358,150 l2/l967 Summer...............

6 Claims,4l )nwing Figures PIIOTOCONDUCTIVE POTENTIOMETER USING VARIABLE TRANSMITTANCE CONTROL STRIPS This invention relates to variable electrical resistors, and more particularly to the use of photoelectric techniques for implementing this type of device.

Variable resistors are used extensively in electrical equipment to implement various control functions. Such variable resistors generally take the form of a potentiometer which comprises a fixed resistive element and a slider arm which can be positioned at any point betweenthe opposite ends of the fixed resistor to provide as an output the portion of the fixed resistance between the slider arm and either end. In a potentiometer, the sum of the resistance between the slider arm and one end and the resistance between the slider arm and the other end remains constant and is equal to the resistance of the fixed resistive element.

In variable resistive devices of the prior art, good electrical contact must be maintained between the slider arm and the fixed resistive element. The friction needed for this contact produces considerable wear, resulting in deterioration of the original properties of the device often causing a break in the fixed resistive element. Further, with wear and oxidation at the contact point between the slider arm and the fixed resistor, the unit will tend to generate electrical noise which may eventually make its utilization intolerable in certain applications. In high vacuum environments, such as encountered in outer space where lubricants evaporate away leaving clean metal-tometal surfaces, the contact may actually cold weld itself to the fixed resistive element, rendering the unit useless.

Prior art potentiometers have a further limitation in that it is often difficult to generate irregular resistive functions especially where unusual tapers are required to represent specific conditions such as, for example, in the case of certain function generators. The production of potentiometers to unusual tapers to represent particular functions often poses a difficult problem involving considerable expense.

The device of this invention overcomes the aforementioned shortcomings of prior art electrical potentiometers by eliminating the use of a sliding contact arm and rather implementing the variable resistive functions by utilizing photoconductive cells, the resistances of which are varied in accordance with the desired resistive functions. As the resistances are controlled by varying the light incident on the photoconductive cells, the desired resistive functions can be implemented by means of optical devices which are photographically produced. The photographic reproduction of both regular and irregular functions is readily accomplished and thus the fabrication of function generators representing irregular functions is greatly simplified and much more economical than with prior art devices.

It is therefore the principal object of this invention to provide an improved variable resistor device using a photoelectric implementation which avoids the use of slider contacts and lends itselfto the reproduction of irregular resistive functions.

Other objects of this invention will become apparent as the description proceeds in connection with the accompanying drawings ofwhich:

FIG. I is a schematic view illustrating the operation of one embodiment of the device ofthe invention,

FIG. 2 is a perspective view illustrating one embodiment of the device of the invention,

FIG. 3 is a plan view illustrating the photographic light control assembly of the embodiment of FIG. 2, and

FIG. 4 is a side elevation view of the embodiment of FIG. 2.

Briefly described, the device of the invention includes a light control element which may be in the form of a strip member interposed between a light source and a photoconductive cell which varies in resistance in accordance with the light incident thereon. The strip member may be in the nature of a photographic wedge having light transmittance variations along its length, such variations being in accordance with the resistive function to be implemented. Such resistive function may be linear, exponential, or may vary irregularly in accordance with a special pattern.

In the case of a potentiometer, two identical strips are utilized in conjunction with separate light sources and photoconductive cells associated with each. The two strips are, in a preferred embodiment, placed alongside each other in a concentric relationship, the light transmittance patterns on the two elements being in opposition to each other. The photoconductive cells and light sources are positioned so that when the two strips are simultaneously rotated, the light incident on one photoconductive cell will increase while the light incident on the other photoconductive cell will decrease in accordance with the resistive function being reproduced, the sum of the light incident on the two cells remaining constant. In this manner, the resistances of the two cells increase and decrease as the light control element is rotated in the same manner as the resistances vary between the control arm and the two ends of a conventional potentiometer.

Referring now to FIG. 1, the operation of one embodiment of the device of the invention is schematically illustrated. Photoconductive cells 11 and 12 are connected together in series so that the sum of their resistances appears between terminals l4 and 15 with the resistance of element ll appearing between terminal 14 and I6, and the resistance of element 12 appearing between terminals 15 and 16. Photoconductive cells 11 and 12 may be any such cells exhibiting a resistance which is in accordance with the light incident thereon. A typical cell suitable for this purpose is a cadmium sulfide (CdS) cell which is commercially available. Light sources 18 and 19 which may be incandescent lamps, are energized by means of power source 22. Light sources 18 and 19 should be identical in their characteristics so that they will provide equal light output.

Interposed between light source 18 and photoconductive cell 11 and light source 19 and photoconductive cell 12 are light control strips 25 and 26 respectively. Light control strips 25 and 26 are units having light transmittance characteristics along their length which vary in accordance with the resistive function to be implemented. Thus, for example, they may be in the nature of light wedges having no light transmittance at one end and full light transmittance at the other end thereof, with a gradation of light transmittance between these two extreme points, such as shown in FIG. 3. This light transmittance could of course also vary in an irregular manner, running from light to dark and back to light again in a particular desired manner representing a special resistive function.

Light control elements 25 and 2 6 are positioned by means of control knob 35 which is coupled to the control elements by means of mechanical linkage 36. The light transmittance characteristics of elements 25 and 26 are identical but the elements are positioned in inverse relationship so that as control knob 35 is actuated, an increase of light transmission results through element 26 while there is a decrease in light transmission through element 25 and vice versa. Thus, it can be seen that as the control knob is actuated, the light incident on cells 11 and 12 varies so as to provide a corresponding variation in the resistance of these two light cells, this variation being such as to maintain the sum of the two resistances constant, the variation being manifested between terminals 14 and 16 and terminals 15 and 16.

Referring now to FIGS. 24, one embodiment of the device of the invention is illustrated. Light control unit 21 which includes control strip elements 25 and 26 is connected to control knob 35 by means of shaft 36 so that it can be rotated by means of the knob. Shaft 36 is rotatably supported in sleeve bearings provided in base 40 and top portion 41 of casing 42. Lamps 18 and 19 are supported in casing top portion 41 and the light is channeled therefrom to strip elements 25 and 26 respectively by means of tube members 46 and 47 which are suspended from casing top portion 41. The light is further channeled from the opposite side of light control strip elements 25 and 26 by means of tube members 50 and 51 which extend from base 40. i

The light from lamp 18 thus is channeled through light control element 25 directly onto cell 11, while the light from lamp 19 is channeled directly through control element 26 onto cell 12, As knob 35 is rotated, the light which is passed through the light control unit 21 is varied, the light incident on one cell increasing while the light incident on the other is decreasing and vice versa. Light control elements 25 and 26 are identical in the light gradients formed around their respective concentric circles but as noted, are arranged in opposition so as to provide the desired increase in the transmittance of one and decrease in the other, so as to maintain the sum of the resistances of the two cells constant.

While strip elements 25 and 26 have been shown in the embodiment of FIGS. 2-4 in a circular configuration, they could also be linearly arranged in an implementation using a linear actuator, following the same principles just described. Also, while the light gradients of elements 25 and 26 have been shown to be substantially linear, these gradients could also be made to represent exponential functions or irregular functions that have unusual increasing and decreasing light transmittance along their lengths, it being readily apparent that these strips can be photographically exposed to represent any desired resistance function. Once a photographic master has been derived, reproductions of this master can easily be made. A master can be readily derived from a curve plotted of the desired resistance function it is to represent by varying the photographic exposure of such master in accordance with this curve.

It is also to be noted that various combinations of the basic resistance elements in parallel, series and series-parallel configurations can be made to provide a great variety of electrical tapers and functions.

The device of this invention thus provides a highly effective implementation for an electrical potentiometer which avoids many of the shortcomings of conventional wiper contact type potentiometers and is more readily adaptable for fabrication to irregular and complicated functions than such prior art devices.

lclaim:

l. A photoconductive potentiometer comprising first and second photoconductive cells,

light source means for providing light to said cells,

a separate light control strip element interposed between said light source means and each of said cells, each of said elements having a variation in light transmittance along its length in accordance with a predetermined resistance function, said strip members being similar but arranged in opposition relationship to each other, and

means for simultaneously positioning both of said strip elements relative to said light source means and said cells so as to vary the light incident on said cells in accordance with said function, said cells being connected together in series, the resistance of one of said cells increasing while the resistance of the other of said cells is decreasing and the sum of the resistances of said cells remaining constant as the position of the elements is varied.

2. The potentiometer of claim 1 wherein said strip elements are arcuate in form and are arranged with one in internal concentric relationship with the other, said positioning means comprising means for rotatably supporting said elements and means for rotatably driving said elements.

3. The potentiometer of claim 1 and further including means for channelling the light from said light source means through said control elements to each of said cells.

4. The potentiometer of claim 2 wherein said strip elements are circular, said cells being arranged on opposite sides of the circles formed by said elements.

5. The potentiometer of claim 1 wherein said strip elements comprise photowedges.

6. A device for providing a resistance which varies as a predetermined function of a mechanical motion comprising:

light source means,

first and second photoconductive cells,

means for controlling the transmission of light from said light source means to said cells comprising first and second light control strip elements arranged in opposing relationship, said strip elements having a variation in light transmittance along their length in accordance with said predetermined function and substantially uniform light transmittance across their widths at any point therealong to provide substantially uniform illumination of the photosensitive areas of the cells, and

means for positioning said strip elements relative to said light source and said cells to vary the light incident on said cells in accordance with said function, said positioning means being adapted to simultaneously position said strip elements together, said cells being connected in series whereby as said positioning means is actuated, the resistance of one of said cells increases while the resistance of the other of said cells decreases, the sum of the resistances of said two cells remaining constant. 

1. A photoconductive potentioMeter comprising first and second photoconductive cells, light source means for providing light to said cells, a separate light control strip element interposed between said light source means and each of said cells, each of said elements having a variation in light transmittance along its length in accordance with a predetermined resistance function, said strip members being similar but arranged in opposition relationship to each other, and means for simultaneously positioning both of said strip elements relative to said light source means and said cells so as to vary the light incident on said cells in accordance with said function, said cells being connected together in series, the resistance of one of said cells increasing while the resistance of the other of said cells is decreasing and the sum of the resistances of said cells remaining constant as the position of the elements is varied.
 2. The potentiometer of claim 1 wherein said strip elements are arcuate in form and are arranged with one in internal concentric relationship with the other, said positioning means comprising means for rotatably supporting said elements and means for rotatably driving said elements.
 3. The potentiometer of claim 1 and further including means for channelling the light from said light source means through said control elements to each of said cells.
 4. The potentiometer of claim 2 wherein said strip elements are circular, said cells being arranged on opposite sides of the circles formed by said elements.
 5. The potentiometer of claim 1 wherein said strip elements comprise photowedges.
 6. A device for providing a resistance which varies as a predetermined function of a mechanical motion comprising: light source means, first and second photoconductive cells, means for controlling the transmission of light from said light source means to said cells comprising first and second light control strip elements arranged in opposing relationship, said strip elements having a variation in light transmittance along their length in accordance with said predetermined function and substantially uniform light transmittance across their widths at any point therealong to provide substantially uniform illumination of the photosensitive areas of the cells, and means for positioning said strip elements relative to said light source and said cells to vary the light incident on said cells in accordance with said function, said positioning means being adapted to simultaneously position said strip elements together, said cells being connected in series whereby as said positioning means is actuated, the resistance of one of said cells increases while the resistance of the other of said cells decreases, the sum of the resistances of said two cells remaining constant. 